h4. THE MISSING LINK BETWEEN THE GROUND LAYER AND THE OVERSTOREY?
Alan L. Yen, Museum of Victoria, GPO Box 666E, Melbourne, Vic 3001
Invertebrates are the animals without backbones – they range in size from microscopic mites to giant spiders, snails and crustaceans that have body weights greater than some species of mammals and birds, yet most are lumped into the category of ‘bugs.’ The involvement of these ‘bugs’ in a diverse range of essential ecological functions does raise the question of what would happen to biological systems if the invertebrates ceased to function.
In Australia, there are some 6,000 species of vertebrates, nearly 100,000 species of described non-marine species of invertebrates, and an estimated further 200,000 undescribed species of non-marine invertebrates (Yen & Butcher 1997). These invertebrates are involved in key ecological functions such as (Yen & Butcher 1997; Majer & Nichols 1998):
* Soil aeration and drainage (eg. ants, termites, earthworms). ). In semi-arid Western Australia, ants have the capacity to make the soil texture profile more uniform by their burrowing and nests. They could affect the whole soil surface in 100 years (Lobry de Bruyn & Conacher 1994). These burrows also aid water infiltration into the soil.
* Leaf litter and woody debris decomposition & nutrient cycling (e.g. springtails, mites, millipedes, oecophorid moths).
* Decomposition of animal remains and products (silphid beetles, dung beetles).
* Pollination (eg. wasps, flies, beetles).
* Seed dispersal and survival (eg. ants, lygaeid bugs). ). While insects can be major predators of some seeds, Berg (1975) estimated that at least 1500 species of Australian plants have food bodies (elaiosomes) on their seeds to attract seed predators in order to assist the dispersal of these seeds away from the parent plant.
* Plant predation (eg. moth larvae, beetles, sap-sucking bugs).
* Maintenance of densities of other animals (eg. spiders, predatory beetles, parasitic wasps).
* Scavengers (eg. ants, fungal feeders).
* Vertebrate food
Habitat complexity can be viewed at several levels. In relation to plants, we can look at the individual plants and its components, or we can consider plants at the community level along with the complex of soil and litter differences at the ground level.
On plants, invertebrates occur on leaves, flowers, stems, trunks, on or under bark, within the tissues of leaves, stems and in tree hollows. These microhabitats provide invertebrates with shelter and food (Majer et al. 1997).
One of the debates about biological systems is whether more complex systems are more stable. While exceptions can be found, it is logical to assume that greater structural complexity results in:
Greater invertebrate faunal diversity (through more microhabitats).
More checks and balances in the system to control imbalances in invertebrate populations.
Habitat simplification can result in significant changes in the composition of the invertebrate fauna. This is due to altered light and temperature regimes, fewer suitable microhabitats, fewer food resources (food plants, floral resources), and loss of habitat and nutrients through greater runoff.
This complexity is even greater when the understorey is taken into consideration. Australia is unique in that it is the only continent that is dominated by two genera of plants: Eucalyptus and Acacia, and one of the characteristics of many Australian forests and woodlands is an eucalypt overstorey with a wattle understorey (Majer et al. 1997).
*IMPORTANCE OF THE UNDERSTOREY*
So how does the understorey contribute to the complexity and stability of biological systems? Physically, it is an important link between the ground layer and the overstorey – some invertebrates use it as the highway between the ground and the overstorey. Biologically it has elements of both the ground fauna and the overstorey fauna, yet it can also have its own unique faunal elements.
Most plant species, whether they are part of the overstorey, understorey or ground layer, have unique assemblages of invertebrates. These assemblages consist of a range of invertebrates that undertake different ecological functions: some are herbivores, others are pollinators, some are predators, parasitoids, etc. A proportion of the herbivores and pollinators are restricted to feed or use one particular species of plant, while others are able to utilise a range of different, but generally related (such as in the same genus or family) plant species. Herbivores often have parasitoids that have strict host specificity, while predators are often less fussy on the type of invertebrate they feed on.
The result is that there is a diverse arrangement of invertebrate assemblages associated with plants – some invertebrate species are restricted to one or a narrow range of plants, while others utilise a wide range. The result is a complex array of invertebrates in the system.
The host plant specificity has important consequences for eucalypt forests and woodlands. Most eucalypt associations consist of several eucalypt species, each with its own assemblage of plant feeders. The level of insect feeding on eucalypts is naturally high (often an average of 20-40% leaf area loss). In some cases, the loss is even greater, resulting in high mortality of trees. Yet in many forests and woodlands, the damage is not uniform across eucalypt species – some species sustain more damage than others. Hence a diversity of eucalypt host species results in a greater diversity of associated invertebrates, and possibly greater stability in the system (Burdon & Chilvers 1974; Morrow 1977).
The wattles also have a rich and diverse invertebrate fauna – often the same groups of insects at the higher taxonomic level, but species with very different evolutionary histories. For example, psyllid bugs are found on eucalypts and wattles – the ones on eucalypts are more closely related to each other, while those on wattles are more closely related. While eucalypt psyllids do not feed on wattles, and vice versa, they may use wattles occasionally for shelter. More importantly, some of their natural enemies probably feed on both eucalypt and wattle psyllids – there is potential for eucalypt and wattle psyllids to maintain higher levels of natural enemies (Riek 1962; Yen 1980; New 1983). These natural enemies have a wider range of food, and may be able to switch from one food source to another depending upon seasonal and/or annual availability. The same situation applies to a range of other plant species in the understorey. A forest or woodland without an understorey or with a simplified understorey, may have higher levels of insect damage in the overstorey.
There is still a lack of research data to back these ideas. There is evidence at the applied level – in the USA, fruit trees are often planted with wattles. The idea is that the natural enemies that feed on wattle insects breed up and move into the fruit trees.
Several other issues need to be kept in mind:
* The importance of gaps in forests or woodlands. Gaps increase light levels and this affects invertebrate activity.
* A diversity of flowering plants results in a greater range of flowering phenologies for use by insects (Woinarski & Cullen 1984; Yen 1989).
*PRACTICAL INVERTEBRATE ISSUES*
It is important to point out that there are major practical differences when working with invertebrates compared to vertebrates and vascular plants:
* Sheer diversity and abundance. The enormous number and diversity of invertebrate species does scare a lot of land mangers. The solution is to restrict the range of invertebrate species for consideration.
* Small body size. Working with invertebrates is easier if species with larger bodies (the so called meso- and macro-invertebrates) are used. However, allowance still has to be made for the different requirements for working with invertebrates compared to vertebrates and plants. For the latter groups, data collection involves collecting and identifying specimens in the field. With invertebrates, most material requires identification using a microscope. The sorting of bulk samples and identification may require an invertebrate worker spending 80% of their working time in the laboratory.
* Taxonomic impediment. The inability to put scientific names on many invertebrates is sometimes cited as a barrier. This can be overcome by using well known groups or identifying specimens to ‘morphospecies’ – specimens that are thought to be the same species are lumped together and given a reference voucher number. For this system to work, it is essential that reference voucher material is deposited into an accessible and permanent institution such as State museums.
* Different life history stages. The fact that the immature stages of many invertebrates bear little resemblance to the adult stage does cause practical problems. This can be aided by, in most cases, restricting the work to adult stages. The main exceptions are aquatic insects, where the immature stages are often the ones found in water, and better known groups such as the caterpillars of butterflies.
The issues of spatial (species turnover) and temporal (seasonality) scale affect plants, vertebrates and invertebrates. The fact that invertebrates are generally so much smaller in body size and that there are so many more species seems to magnify the issue of scale. With regard to spatial scale, I can only recommend that that land managers to aware of the fact that invertebrate research is generally based on a small scale (plots) and management is based at the larger (landscape) scale. The use of larger, often ecological, units of classification such as trophic levels, guilds, and functional groups, is one way that could be used to overcome large rates of turnover at the species level. Do not expect that plant communities provide an appropriate umbrella for invertebrates (Yen 1987).
*WHAT IS REQUIRED?*
I am speculating that we have underestimated the importance of the understorey for invertebrates as part of the ecology of forests and woodlands. In the management of remnant vegetation and in the restoration of disturbed systems, the importance of the understorey and its associated invertebrate fauna is often forgotten.- yet they are all part of a complex and dynamic system. In order to understand and management this component, we require:
* More information. Surveys to document the invertebrate assemblages on overstorey, understorey and ground layer plants in one or more selected vegetation communities.
* Research on possible interactions between the overstorey, understorey and ground layer invertebrate assemblages in these vegetation communities.
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